1. Field of the Invention
This invention relates to a disposable syringe, more particularly to a disposable syringe which enables a needle cannula to be retracted within a barrel after use for safe disposal.
2. Description of the Related Art
In co-pending U.S. patent application Ser. No. 10/143,461, the applicant disclosed a disposable syringe that includes a barrel, a tubular needle seat, and a plunger. The barrel has an inner surrounding barrel wall surface which confines a passage and which includes a larger-diameter segment and a smaller-diameter segment that are disposed proximate to lower and upper open ends, respectively, so as to confine a surrounding shoulder portion therebetween. The tubular needle seat is insertable into the passage, and includes a lower surrounding edge portion which abuts against the surrounding shoulder portion, and a surrounding seat wall which has an inner tubular wall surface that has a grip segment, and an outer tubular wall surface that includes a lower segment retained at the smaller-diameter segment by virtue of a first friction force, and an upper segment adapted to be sleeved over by a needle hub of a needle unit. The plunger includes a stem portion which is movable in the passage of the barrel and which has an outer segment that extends outwardly of the lower open end of the barrel, a surrounding engaging portion which is retainingly sleeved on an inner segment of the stem portion by virtue of a second friction force and which is in sealing contact with and which is slidable relative to the larger-diameter segment so as to be moved with the stem portion, and a head which extends from the inner segment toward the grip segment. As such, when the surrounding engaging portion is brought by the inner segment to engage the lower surrounding edge portion, and is depressed by the lower surrounding edge portion against the second friction force by a manual force which is generated as a consequence of continuing movement of the inner segment, the surrounding engaging portion is retained by the lower surrounding edge portion to thereby permit the head to move towards the grip segment of the tubular needle seat and to be held by virtue of a fourth friction force that is greater than the first friction force. Therefore, the used needle unit can be enclosed in the passage of the barrel for safe disposal.
It is desirable to improve the aforesaid disposable syringe to simplify the construction thereof and to be adapted for use in conjunction with needle units of various forms.